Cracking furnace having thin straight single pass reaction tubes

ABSTRACT

THE PRESENT INVENTION CONCERNS A METHOD AND APPARATUS FOR TREATING A HYDROCARBONACEOUS PROCESS FLUID BY INDIRECTLY HEATING SUCH FLUID TO HIGH TEMPERTURES IN EXTREMELY SHORT PERIODS OF TIME AND THEN RAPIDLY COOLING. THE PROCESS FLUID IS PASSED INTO THE STRAIGHT SINGLE PASS REACTION TUBES OF THE NOVEL HEATER OF THIS INVENTION, SUCH REACTION TUBES BEING CONTAINED IN A REFRACTORY ENCLOSURE. THE TUBES ARE EACH CONNECTED TO AN INDIVIDUAL QUENCH TUBE WHEREIN THE PROCESS FLUID MAY BE RAPIDLY COOLED EITHER BY A COOLANT FLOWING THROUGH A PLURALITY OF JACKETS   EACH SURROUNDING EACH INDIVIDUAL QUENCH TUBE OR, IN ANOTHER EMBODIMENT, THE PROCESS FLUID MAY BE COOLED BY HAVNG THE QUENCH TUBES IMMERSED IN A LIQUID BATH. IN ANOTHER EMBODIMENT OF THIS INVENTION A METHOD AND APPARATUS IS PROVIDED FOR CARRYING OUT THE AFOREMENTIONED METHOD WHILE SIMULTANEOUSLY DECOKING INDIVIDUAL REACTION TUBES.

June 20, 1972 B. A. WALLACE PASS REACTION TUBES 5 Sheets-Sheet 1 FlledJune 15, 1970 I I F W L %a m mm? n ll l |H .l II v H fi A? will M AH M.4 I M P ll|l l W E- l WW L l. I Q g I I 5: E 1313:; W W M II H L H HI;fl "I H I A A M 1|1 I; I H 4 4 L3 u 3 I a Q l H 2 i a 00 lwlw /0 H r 115 H J? .f/ 7/ c F r g U1 I B M 1 M e My mm mw 2 3. 2% Ac M IW M I 5 June1972 B. A. WALLACE 3,671,198

CRACKING FURNACE HAVING THIN STRAIGHT SINGLE PASS REACTION TUBES FiledJune 15, 1970 5 Sheets-Sheet 2 34 szj FIG. 2

f4 INVENTOR.

Bruce ,4 h/affaae BY Am (1. QWZMkW Armmir MHEfSQW June 20, 1972 B. A.WALLACE 3,671,193

CRACKING FURNACE HAVING THIN STRAIGHT SINGLE PASS REACTION TUBES F1160June 15, 1970 5 Sheets-Sheet 5 fi \K .52

INVENTOR.

62 "50 BY r47 OR/Vf) June 20, 1972 B. A. WALLACE 3,671,193

CRACKING FURNACE HAVING THIN STRAIGHT SINGLE PASS REACTION TUBESPatented June 20, 1972 3 671 198 CRACKING FURNACIi HAVING THIN, STRAIGHTSINGLE PASS REACTION TUBES Bruce Alden Wallace, White Plains, N.Y.,assignor to Ppllman Incorporated, Chicago, Ill.Ctlllllitl60nil;16-17J3I%flf applilclation Ser. No. 683,703,

0v. s a 'cati J 15 SerinNo. 46,043 Pp on une 1970, t. Cl. B01j 3/00;C101) 1/04; C 9/04 US. Cl. 23-277 g 10 Claims ABSTRACT OF THE DISCLOSUREThe present invention concerns a method and apparatus for treating ahydrocarbonaceous process fluid by indirectly heating such fluid to hightemperatures in extremely short periods of time and then rapidlycooling. The process fluid is passed into the straight single passreaction tubes of the novel heater of this invention, such reactiontubes being contained in a refractory enclosure. The tubes are eachconnected to an individual quench tube wherein the process fluid may berapidly cooled either by a coolant flowing through a plurality ofjackets each surrounding each individual quench tube or, in anotherembodiment, the process fluid may be cooled by having the quench tubesimmersed in a liquid bath. In another embodiment of this invention amethod and apparatus is provided for carrying out the aforementionedmebthod while simultaneously decoking individual reaction to es.

This application is a continuation-in-part of copending application Ser.No. 683,703, filed Nov. 16, 1967 and now abandoned.

The present invention relates to an improved method and apparatus fortreating hydrocarbonaceous process fluids. The method and apparatus ofthe invention is particularly adapted to carrying out rapid hightemperature heating of process fluids, such as is required in modernhydrocarbon pyrolysis whereby hydrocarbon feedstocks such as ethane,propane, naphtha or gas oil may be thermally cracked to produce lesssaturated products such as acetylene, ethylene, propylene, butadiene,etc., and other products such product mix depending, for example, on thenature of the feedstock.

Heretofore, prior processes for pyrolyzing hydrocarbons by indirectheating have been limited by the process fluid eflluent temperatureattainable. In the extreme, in indirect heating processes, the processfluid has been heated to temperatures as high as 1650 F. It was foundthat if such hydrocarbonaceous process fluids were subjected to highertemperatures for the residence times heretofore employed in suchprocesses, i.e., 0.1 or more seconds, portions of the process fluidwould be degraded to gummy polymeric substances and carbon, usuallycollectively referred to as coke. Coke formation within indirectlyheated tubes cuts down the heat transfer to the process fluid therebyincreasing the temperature of the tube walls and also increases thepressure drop of the fluid. Further, it was discovered that thecombination of high temperatures and long residence times drasticallyincreased the quantity of light, saturated hydrocarbons produced. Thepresence of large quantities of such products substantially increasedthe difficulties in recovering the desired olefin products from thepyrolysis eflluent and further, reduced the yield of olefins for a givenquantity of process fluid charged to the heater.

Another factor limiting efl'luent temperature in prior processes is thelength of reaction tubing which must be provided to permit the entirevolume of fluid passing therethrough to be heated to the requisitetemperature.

In order to obtain high furnace capacity the process tubes are usuallyof three to five inch inside diameter; a relatively long fired tube(about to 400 feet) is required to heat a three to five inch diameterfluid mass to the requlred temperature and the prior art furnaces thusrequire a coiled or serpentine tube configuration to fit the requiredlength of tubing into the confines of a reasonably sized furnacerefractory enclosure. The problems of coke formation and pressure dropacross the furnace tubes are aggravated by the multiple turns of acoiled process tube configuration. Inasmuch as the total number of molsof gaseous material increases during the pyrolysis of hydrocarbons toethylene and other olefins, this particular process is carried out atabout atmospheric pressure since elevated pressures would tend to reducethe extent of reaction and adversely affect the product yield. Otherprocesses such as, for example, steam re-forming are advantageouslycarried out at elevated pressures. Therefore, it is desirable in caseswhere low or atmospheric pressure operation is desired, to minimize thepressure drop through the furnace, since overcoming a high pressure dropwill require a correspondingly high effective pressure in the furnacetubes. In addition to the pressure-drop problem they pose, maintenanceand construction costs of serpentine coils are also higher than would bethe case of straight run tubes, but, as mentioned above, the coiledtubes are necessary to limit the size of the furnace enclosure toreasonable proportions. Accordingly, prior processes have been limitedto the aforementioned temperatures when heating the process fluidindirectly.

Other processes have been suggested which provide means for heating theprocess fluid to temperatures higher than those heretofore attained byindirect heating in tubes. These processes generally comprise combustinga fuel in the presence of an oxygen containing gas and then intermixingthe resulting hot products of combustion with the hydrocarbonaceousprocess fluid to be treated. High temperatures are attainable inrelatively short periods of time and the problems normally associatedwith the resultant coking, e.g., decrease in heat transfer and cloggingof indirectly heated tubes are generally not applicable to processes ofthis type which eliminate the need for such indirect heating in tubes. Amajor drawback, however, is that the products of combustion leave thereactor 1n admixture with the desired product and add to the diflicultyof recovering such products.

In order to fix the product composition by halting the reactions of theheated products as they emerge from the reaction tubes, the processfluid must be cooled quickly to a temperature below reactiontemperature. This is usually accomplished by cooling the fluid eitherdirectly or indirectly, or both, with a coolant liquid, usually water inthe case of indirect cooling, and water or a refractory oil or acombination of water and oil in the case of direct cooling. Because ofthe high temperature of the reactants, high pressure steam can bereadily generated from the coolant water. The thermal stresses of suddencooling and the diversion of high velocity, extremely hot reactantstreams from the tube exits into the indirect cooling ex changers(usually referred to as quench boilers) entails severe stresses on thematerial of the tube exits and the quench boilers themselves, andconcomitant design problems. The problem is complicated by the necessityof Holding the transition period, i.e., the time period from the timethe fluid leaves the cracking zone of the reaction tubes and until itreaches the cooling zone of the quench boiler, to a minimum so as tominimize the amount of reaction during the transition period. Rapidcooling and minimizing the transition period, during which undesirableside reactions occur, is necessary in order to freeze the chemicalcomposition of the fluid at the composition attained at the positivelycontrolled temperature of the reaction zone outlet.

-It is therefore an object of this invention to provide a method fortreating hydrocarbonaceous fluids by heating them to high temperatureswithout the drawbacks heretofore encountered in prior processes.

It is another object of this invention to accomplish the above objectand to provide a method for rapidly cooling the heated fluid belowreaction temperature thereby fixing the effluent product composition.

It is a further object of this invention to provide a simple, efficientand relatively inexpensive furnace capable of carrying out the foregoingmethod with minimum downtime for maintenance.

These and other objects of the invention will be apparent from thefollowing description and accompanying drawings.

The method of the invention comprises passing a hydrocarbonaceousprocess fluid through a plurality of essentially straight reaction tubescontained in a refractory enclosure. Heat is transferred, by radiationand convection to the outside walls of such reaction tubes, byconduction through the tube walls, and by convection to the processfluid. Such heat is supplied by burners which combust a suitable fuel.The process fluid is heated in this manner to a temperature which mayrange from about 1550 to about 1850 degrees Fahrenheit. Such heating maybe accomplished in a residence time of about 0.2 to about 0.01 second.The heated process fluid is then passed through a plurality of quenchtubes each connected in flow communication with a corresponding reactiontube and is rapidly cooled therein. Such cooling may be accomplished byindirectly exchanging heat with a coolant coursing through a jacketsurrounding each individual quench tube. Alternately, such cooling maybe accomplished by passing the heated fluid through the individualquench tubes which may be partially submerged in a liquid bath common toall the quench tubes. In this manner the heated fluid passes through thetubes and i first partially cooled by exchanging heat indirectly withthe bath through the walls of the tubes. Then the partial- 1y cooledfluid is further cooled directly by passing out of the quench tubes andthrough the liquid bath. Preferably, the liquid bath comprises moltenlead. Other substances may be used to achieve lower melting point orvapor pres sure characteristics.

In the case of the pyrolysis of a hydrocarbonaceous process fluid, theprocess fluid is passed through the straight reaction tubes and heatedtherein by heat supplied from the burners to temperatures ranging from1550 to 1850 F., and preferably from 1650 to 1750 F. The process fluidmay have been preheated to less than reaction temperatures in apreheating coil. (The preheating coil may advantageously be heated 'byflue gases exhausted from the refractory chamber.) Upon attaining thereaction temperature the desired reactions occur and the fluid ismaintained within the reaction temperature zone for the requiredresidence time which is about 0.1 to about 0.01 second and preferablyabout 0.10 to about 0.02 second. Residence time within any segment ofthe reaction tubes is of course simply controlled by controlling thelinear velocity of the fluid.

It has been discovered that by limiting the residence time within theabove described ranges, the process fluid may be heated to the highaforementioned effluent temperatures without substantial degradation tocoke. Further, it has been discovered that by heating within the shortresidence times prescribed by this invention, a low production of light,saturated hydrocarbons result, the high etfluent temperaturesnotwithstanding.

Still another advantage accrues by using the method of this invention.It has been found that by operating within the prescribed ranges, asubstantially larger proportion of the feed is converted into olefins.In particular there is a 4 substantial increase in the yield of olefinsheavier than ethylene.

As aforementioned, it is necessary to rapidly cool the furnace efiiuentsin order to arrest the reaction and fix the product composition.Accordingly the reacted products pass through the portion of the tubesWithin the refractory enclosure, i.e., the reaction tubes, and into thequench tubes wherein they are cooled, and the cooled products are thenwithdrawn from the quench tubes. The provision of individual quenchtubes directly connected to each process fluid tube greatly simplifiesthe difficult design problem of collecting hot, cracked product gas athigh velocity from a plurality of reaction tubes and diverting them intoa separate quench boiler. It likewise drastically reduces the time lagbetween the time the fluid leaves the reaction tube and the time itenters the quench tube, thereby precluding degradation or decompositionof the valuable products by undesirable side reactions (which occur, asaforesaid, during the interval between the heated reaction tube and thecooled quench tubes).

Inasmuch as the reaction tubes are straight and contain no loops orcoils, the length of reaction tube necessary to heat the fluid to therequired temperature sets the minimum size of the combustion chamber. Inorder to limit this size to a practical and economical dimension,considering structural requirements and sound engineering practices, thediameter of the reaction tube must be limited so that the volume offluid passing therethrough can be sufliciently heated during itsresidence time by the available heat input, which in turn is limited bythe maximum allowable wall temperature and heat flux capacity of thetubes. Since the volume of fluid passing through the tube at a givenlinear velocity is proportional to the square of the tube insidediameter, a furnace designed in accordance with the prior art with, forexample a foot long, 4" inside diameter tube (coiled to fit within areasonable size furnace) would have the same fluid capacity as a furnacedesigned in accordance with the present invention which is equipped withsixteen one-inch inside diameter tubes forty feet in length; i.e., thecross sectional area of the single four-inch inside diameter tube is thesame as the sum of the cross sectional area of the sixteen one-inchinside diameter tubes. Since each one-inch tube is carrying butone-sixteenth the fluid carried in the four-inch tube, yet hasone-fourth as much heated surface area per unit length, only aboutone-fourth the total heated length is required to impart an equivalentamount of heat to the fluid. Therefore, a furnace in accordance with thepresent invention with sixteen one-inch tubes, forty feet in length(one-fourth of 160) provides about the same furnace capacity, both interms of fluid throughput and heat input, as a prior art furnace with asingle four-inch tube, 160 feet long.

In the case of furnaces designed to carry out hydrocarbon pyrolysis toobtain primarily ethylene as well as other products, for example,current engineering feasibility limits the size of the furnacerefractory enclosure to about 60 feet, more or less. Accordingly, thelength of the single pass reaction tubes is likewise limited to about 60feet more or less, and consequently, the mass of fluid being heated inthe reaction tubes must be limited to enable the fluid to attain therequired pyrolysis temperature within the short residence time requiredfor satisfactory ethylene yield and within the limitation of the maximumsubstainable tube wall temperature. It has been found that with a tubelength of not more than about 60 feet, the required temperature forpyrolysis of hydrocarbon feedstocks in the service described above, canbe attained in the hydrocarbon fluid mass if the hydrocarbon fluid massdoes not exceed about two inches in diameter, i.e., if the reaction tubehas a maximum inside diameter of about two inches. To state itdifferently, it has been found that in this particular service, a ratioof tube inside diameter to reaction tube length of not more than about1:360 (2 inches/ 60 feet= will permit efficient operation. Asignificantly greater inside diameter-to-length ratio would precludesuflicient heat being imparted to the fluid within the residence timeand tube wall temperature limitations and result in failure to attainthe desired degree and extent of thermally-induced chemical reactions.It is apparent that if a fixed tube length, maximum allowable tube walltemperature, and fixed residence time are stipulated the amount of heatwhich can be imparted to the fluid mass decreases as the diameter of thefluid mass increases.

Of course, the ratio between the maximum permissible tube insidediameter and the tube length will depend upon the type of servicerequired and the feedstock to be treated in the furnace. A greater orlesser furnace dimension may "be feasible in other services thuspermitting a greater or lesser tube length and corresponding tubediameter. In most cases, the maximum permissible tube inside diameterwill not be greater than about three inches. Generally, the ratio ofinside diameter to length should not exceed The residence time andtemperature and heat input requirements will likewise vary, within theranges specified herein, depending on the service in which a furnace isemployed. It is apparent that it is within the ordinary abilities of oneskilled in the art, given the disclosure of the present invention, todetermine the particular requirements of the service in which thefurnace is to be used and to size the furnace and the tubes for aparticular service so as to obtain an eflicient furnace designed inaccordance with the present invention and particularly adapted to theservice in which it is to be used. It is further apparent that suchfired heaters are contemplated by the present invention.

It is thus apparent that a furnace designed in accordance with thepresent invention provides significant advantages over furnaces designedin accordance with the prior art. For example, the short reaction tubelength permits extremely short residence times in the process tubeswhich has the important process advantage in producing ethylene andother olefins by hydrocarbon pyrolysis, of increasing the yield ofethylene from the feed. Shortened residence times in the quench tubesand in the transition between the reaction and quench tubes are alsoattained. The straight-tube design reduces pressure drop significantlyas compared to coiled or serpentine process tubes. The straight reactionand quench tubes permit efiicient and highly effective decoking by meansof high pressure steam or other fluid lancing during shutdown oron-stream decoking and, during operation of the furnace, a largequantity of fluid (preferably steam) may be injected into any individualreaction tube by means of valved, flexible fluid (steam) injection hoseswhich can be connected to any of the furnace reaction tubes. Theflexible hose connections are located downstream of metering orificeslocated in the process fluid inlet end of each reaction tube. Themetering orifices are high pressure drop orifices which provide uniformdistribution of process fluid into each of the reaction tubes. Injectionof a large quantity of decoking fluid, e.g., steam into the reactiontube downstream of the metering orifice during furnace operationprovides the tube with a large slug of steam in addition to processfluid; as a result heat is absorbed at a faster rate by the combinedvolume of steam and process fluid and the tube being decoked is cooledsufliciently to cause it to contract and thereby spall off cokeparticles from the tube inside surface. When the steam injection isstopped, the tube heats up and expands, providing an additional spallingeflect. Decoking may be accomplished by injecting other fluids thansteam, such as, for example, water. The spalled coke particles pass withnaturally occurring particles in the process fluid to the furnace outletand are removed in the usual scrubbing and fractionating steps practicedon the furnace eflluent. Because of the enhanced scouring effect and theelimination of turns in the tubing, steam or other fluid lancing, eitheralone or with the addition of scouring granules, quickly and efiicientlycleans the tubes so that an individual tube may be efficiently decokedwhile the rest of the unit stays in service. It will be appreciated thatthe feature of on-steam decoking is an extremely advantageous one whichwill greatly reduce the number and length of expensive shutdowns of theentire furnace.

A small diameter reaction tube has the advantage of having smallermechanical forces acting on it from the pressure of the process fluidthan does a large diameter tube. Therefore, the tube wall thicknessnecessary to contain the process fluid mass within the tube need not beas great in a small diameter tube as in a large diameter tube. Thispermits a considerable reduction in the quantity of expensive heatresistant alloys required for the tubes as compared with an equivalent,large diameter tube furnace. Another significant advantage of thefurnace of the invention resides in the fact that the relatively largenumber of small diameter tubes (as compared to the small number of largediameter tubes of furnaces designed in accordance with the prior art)means that any given tube carries a much smaller percentage of the totalfurnace throughput (tube capacity varies with the square of thediameter). Accordingly, the failure of a tube does not seriously affectthe total furnace throughput and the failed tube may be isolated byvalves or other shutoff devices and the furnace kept on stream. Withfurnaces operated at or near the metallurgical limits of availablematerials of tube construction, it will be appreciated that readyisolation of failed reaction tubes with but small diminution of furnacecapacity is a valuable feature of the invention.

Yet another advantage of the straight-tube, relatively small diameterfurnace is that a pilot plant installation may conveniently consist of asingle tube from a commercial unit so that data obtained in the pilotunit can be exactly reproduced in commercial scale; the commercial unitconsists merely of a large number of fluid tubes identical to those usedin the pilot unit. For example, if a typical furnace coil of the priorart contains a given number of serpentine coils of feet of reaction tubelinear length and 4 inch inside diameter, an equivalent furnace designedin accordance with the present invention would contain the same givennumber of groups of sixteen reaction tubes of one-inch inside diameterand forty feet of reaction tube length. It is obviously not feasible toconstruct a pilot unit with a 160 foot, 4 inch inside diameter tubecoil, while it is entirely feasible to construct a pilot unit containinga single forty foot, one-inch tube; furthermore, the use of the one-inchpilot tube would require diversion and disposal of but one-sixteenth theamount of test fluid as would be required in the four inch tube. It isapparent that for a furance tube designed in accordance with the priorart, the pilot unit must be a scaled-down version of the commercial unitand this will require complex calculations and empirical assumptions inconverting the pilot plant data to predict commercial performance;whereas a pilot unit for a furnace tube designed in accordance with thepresent invention consists of one or more tubes identical to those whichwill form the commerical unit and the data obtained therefrom directlyestablishes the performance of the commercial unit. The assumptions andcalculations involved in scaling-up data, which cast grave doubt on thevalidity of scaled-up data arid often require that a commercial unit beover-designed to insure meeting its specifications, are thus entirelyavoided by the present invention.

The invention will be more clearly understood from the followingdescription and drawings of preferred embodiments of the invention whichare illustrative of fired heaters designed in accordance with theinvention.

FIGS. 1, 2 and 3 of the drawings show, respectively, a side elevation,an end elevation and an isometric view, all in partial section, of apreferred embodiment of the invention.

'FIG. 4 is a schematic representation of a preferred embodiment of azoned furnace constructed in accordance with the invention.

FIG. 5 is a schematic representation of the arrangement of analternative preferred embodiment of the invention.

Referring now to FIGS. 1, 2 and 3 which illustrate a preferredembodiment of the invention, and particularly to FIG. 3, a refractoryenclosure encloses reaction tubes 12 which extend past the enclosure asquench tubes 14. Reaction tubes 12 and quench tubes 14 are coaxiallyaligned and connected in flow communication with each other. Support forthe entire structure is provided by steel framework 1. Each of quenchtubes 14 is jacketed by cooling jackets to constitute quench coolers 16.As best shown in FIG. 2 a coolant liquid (preferably water) header 18provides coolant to the quench coolers 16 via coolant connectors 20.Vapor (steam) is generated within the quench coolers by absorbing heatfrom the quench tubes and the vapor and recirculating liquid passesthrough vapor connectors 22 into vapor drum 24. Make-up liquid (water)is introduced through line 26 into vapor drum 24 thence to coolantheader 18 via downcomer lines 30. Uncondensed vapor (steam) is taken offvia line 28.

Process fluid is introduced to the furnace via line 40, preheated inpreheat coil 42 and the preheated fluid is passed via lines 44A and 44Bto process fluid headers 46, thence via flexible process fluidconnectors 48 into reaction tubes 12 wherein the fluid is heated to thetemperature required to have the desired chemical reaction and/ orchange of state take place. Metering orifices 49 are located in eachfluid connector 48 for the purpose of distributing the process fluiduniformly to the reaction tubes by maintaining a high pressure drop (inrelation to the pressure drop through the furnace) across the orifices49. The reacted fluids pass directly into quench tubes 14 wherein theyare cooled sufliciently to halt the reaction and freeze the compositionof the product mix. Quenched, reacted products then pass through productconnectors 50 to product header 52 from whence quenched product isremoved via line 54.

Heat is provided to the furnace by the combustion of a suitable fuel inburners 56. Fuel and air is supplied to the burners via lines 58 throughsuitable connections (not shown). Flames from burner 58 provide directradiant heating for reaction tubes 12. Hot combustion gases, which arewithdrawn from enclosure 10 via flue 60, are used to preheat the processfluid in preheat coil 42, after which further heat may be recovered fromthe gases in coil heater 43 (FIG. 1) for example, by superheating steam,which is introduced into heater 43 via line 45 and withdrawn via line47. The cooled gases are then withdrawn from the furnace.

Steam for decoking the process fluid tubes is provided from decokingsteam header 32 to valved connection 34 which provides connection viaflexible steam hose 36 to a number, about eight, of tubes 12, which canbe serviced by each valved connection 34. Removable coupling 38 andsteam inlet valves 39 permit steam hoses 36 to be readily connected anddisconnected between the several tubes serviced by each of the pluralityof steam hoses. By the present arrangement, decoking steam may beinjected into the individual reaction tubes in need of decoking whilethe furnace stays on stream. When the furnace is shut down for regularmaintenance, high pressure steam or water lances may be introducedthrough steam inlet valves 39 to permit additional cleaning of thetubes. Suitable drainage plugs 62 are provided at the bottom of eachprocess fluid tube to drain ofl decoking steam or water and coke removedfrom the inside surface of the tubes during such shutdown cleaning.During on-stream decoking, the decoking steam and removed coke particlesmerely pass out of the furnace with thefurnace effluent.

Details of the arrangement of the feed inlet and onstream decokingconnections, burners and tube supports are shown in end view in FIG. 2and in isometric view in FIG. 3. In order to simplify the drawing onlyrepresentative elements are shown; the close spacing of the elementswould needlessly complicate the drawing if each element were portrayed.Each tube is flexibly supported by tube support cables 64 strung overpulleys 66A and 66B and held by counterweights 68. Each tube is thusflexibly supported to allow for thermal expansion during operation ofthe furnace. The various connections and supports are sheltered from theelements by shed enclo sure 70.

In another preferred embodiment of the invention the design may beadapted to a zoned furnace, i.e., a furnace in which sections of thereaction tubes are physically isolated from one another and providedwith individual sets of burners. In this manner, by firing the burnersat different rates, the heat input to each section of the reaction tubescan be controlled to provide a further refinement of control over thetime-temperature profile.

FIG. 4 shows in schematic partial elevation a preferred embodiment of azoned furnace designed in accordance with the present invention in whicha steel supporting framework supports the refractory enclosure 101 whichcontains inward sloping walls 102 which divide the refractory enclosureinto an upper radiant section 104A and a lower radiant section 104B. Thelower radiant section 1043 is fired by floor burners 106 which heat thelower portion of reaction tubes 108. Combustion gases from burners 106flow upwardly and are channeled by sloping walls 102 into flue gas duct110 within which is disposed convection pre-heating coil 112. Thecombustion gases are withdrawn from the furnace via duct 110.

In like manner, roof burners 114 fire upper radiant section 104A andheat the upper portion of reaction tubes 108. The combustion gases arewithdrawn through flue gas duct 110 in which they are comingled with thecombustion gases from floor burners 106.

Process fluid is introduced via line 116, preheated by convectionheating in coil 112, the preheated fluid is passed via line 118 toprocess fluid header 120, thence through metering orifices 121, thencethrough reaction tubes 108 serially through lower radiant section 104Band upper radiant section 104A of the refractory enclosure, whichsections may be fired at different rates, thence into quench tubes 120.The hot reacted gases are cooled within quench tubes 120 by a coolingmedium, usually Water, introduced from coolant header 122 via coolantconnectors 124 and through quench coolers 126. The cooling medium isvaporized and the vapor withdrawn through vapor connectors 128 intoboiler outlet header 130, thence to a vapor drum (not shown). Thereacted, quenched process fluid is removed via product connectors 132and product header 134.

On-stream decoking of individual tubes 108 is accomplished byintroducing decoking steam from decoking steam header 136 via flexiblesteam hose 138 into reaction tubes 108 downstream of metering orifices121. For shutdown cleaning of the tubes, high pressure steam or waterlances may be connected at the upper end 109 of tubes 108 and fluid andcoke removed via drainage plugs 140.

Each reaction tube is flexibly supported to allow for thermal expansionduring operation by means of support spring 142 and yoke 144.

Yet another preferred embodiment of the present invention is the use ofstraight relatively small diameter reaction tubes utilizing aU-connection to the quench tubes so that the total furnace dimension(height in this case) is set by the required length of the reactiontubes only, and not by the combined length of reaction and quenchtubing. This embodiment thus reduces the overall furnace height, but atthe expense of the added pressure drop, cost and inconvenience ofintroducing a U-turn into the tubes. FIG. 5 shows in schematic partialelevation such a preferred embodiment of a furnace in accordance withthe present invention consisting of a steel framework 200 supportingrefractory enclosure 202 which encloses burners 204 and reaction tubes206. U-connectors 208 connect reaction tubes 206 to quench tubes 210which are encased by quench coolers 212. Cooling medium is introducedinto the quench coolers via lines 214A and 214B, partially vaporized inquench coolers 212 and the vaporized and recirculating coolant iswithdrawn via vapor connectors 216 to vapor drum 218 from whence vaporsare withdrawn via line 219.

Process fluid is preheated, preferably in a convection preheat coil (notshown) by combustion gases from the furnace, and is passed via line 220into process fluid header 222 thence through orifice meters 223, thenceinto reaction tubes 206. The heated reacted products pass throughU-connections 208, are cooled in quench tubes 210 and withdrawn viaproduct header 224 and line 226.

On-stream decoking of individual reaction and quench tubes isaccomplished by introducing decoking steam from decoking steam header228 via flexible steam hoses 230 and valved connections 231. The steamis introduced downstream of orifice meters 223. For shutdown decoking,high pressure steam or water is introduced through Y-connectors 209 anddrainage is provided by drainage plugs 232 which service reaction tubes206 and quench tubes 210. Tube supports and other structural elementsare omitted from the drawing of FIG. to simplify the drawing.

To further illustrate the methods of this invention the followingexample is given.

A gas oil feed is first combined with steam and then passed throughstraight, small diameter, indirectly heated tubes. The steam to oilratio and the outlet temperature are held substantially constant for tworuns; however, the residence time is varied from a relatively slow timeto the time prescribed by this invention. The table below illustratesthe operating conditions and the results.

Examination of the table reveals the advantages of operating inaccordance with the methods of this invention. Run 2, wherein theresidence time was 0.066 second in contrast to a 0.233 second residencetime for run 1, results in superior feedstock utilization whilemaintaining the ethylene production at substantially a fixed level.There is a markedly decreased tail gas production, i.e., the weightpercent yield per pass of hydrogen, methane and acetylene has beenreduced from 16.48 percent to 11.66 for runs 1 and 2, respectively.Moreover, increased propylene and butadiene and butene yields'arerealized.

In the above description and the attached drawings numerous valves,pumps, meters, etc. necessary or useful in operating the fired heatersdescribed have been omitted for the sake of clarity; such items andtheir use are well known to those skilled in the art. The invention isnot intended to be limited by the preferred embodiments de- 10 scribedin detail and it will be apparent to those skilled in the art thatnumerous modifications to the embodiments described are possible withoutdeparting from the scope of the invention.

What is claimed is:

1. A fired heater comprising:

(a) a refractory enclosure of limited length;

(b) a plurality of essentially straight pass reaction tubes containedwithin said limited length refractory enclosure and each having a lengthof up to about 60 feet and an inside diameter of not more than one twohundred and fortieth 5 of the length of the respective reaction tube;

(c) means to heat said reaction tubes;

(d) a plurality of quench tubes, connected in flow communication withsaid reaction tubes;

(e) means adapted to course a process fluid through said reaction andquench tubes; and

(f) means adapted to cool said process fluid after leaving said reactiontubes.

2. The fired heater of claim 1 wherein each reaction tube is connectedin flow communication with a quench tube and the means adapted to coolthe process fluid comprise a plurality of cooling jackets each adaptedto course coolant over the external surface of each of said quenchtubes.

3. The fired heater of claim 1 wherein the means adapted to cool theprocess fluid comprise a liquid bath adapted to surround each of thequench tubes.

4. A fired heater comprising:

(a) a refractory enclosure of limited length;

(b) a plurality of essentially straight single pass reaction tubescontained Within the refractory enclosure and each having a length of upto about 60 feet and an inside diameter of not more than one two hundredand fortieth (V of the length of the respective reaction tube;

(0) means adapted to heat said reaction tubes;

(d) a plurality of essentially straight quench tubes connected in flowcommunication with said reaction tubes;

(e) means adapted to course a process fluid through said reaction andquench tubes;

(f) means adapted to cool said process fluid after leaving said reactiontubes; and

(g) means to inject decoking fluid into any reaction tube while saidheater remains in service.

5. The fired heater of claim 4 wherein the length of said refractoryenclosure of limited length is less than sixty feet.

6. The fired heater of claim 4 wherein each reaction tube is connectedin flow communication with a quench tube and the means adapted to coolthe process fluid comprise a plurality of cooling jackets, each adaptedto course coolant over the external surface of each of said quenchtubes.

7. The fired heater of claim 4 wherein the means adapted to cool theprocess fluid comprise a liquid bath adapted to surround each of thequench tubes, each quench tube being at least partially immersed in saidbath so as to have the cooling process fluid effluent end of said quenchtube below the liquid level of the bath.

8. A fired heater for pyrolyzing normally gaseous or normally liquidaromatic and/or aliphatic hydrocarbon feedstocks to obtain olefins andother products compris- (a) a plurality of essentially straight,vertical, single pass reaction tubes each hawng an inside diameter notgreater than 3 inches and a length between 40 and 60 feet.

(b) a refractory enclosure containing said tubes and fitted with burnermeans to heat said tubes;

(c) a plurality of quench tubes connected in flow communication withsaid reaction tubes;

(d) means adapted to course process fluid through each paired reactionand quench tubes; and

(e) means to inject decoking fluid into any reaction tube while saidheater remains in service.

9. A fired heater comprising:

(a) a refractory enclosure of limited length;

(b) a plurality of essentially straight single pass reaction tubescontained within said refractory enclosure, each of said reaction tubeshaving a length of up to about 60 feet and an inside diameter of notmore tharr'one two hundred and fortieth of the length of its respectivereaction tube;

(c) a chamber containing quenching fluid; and

(d) a plurality of quench tubes connected in flow communication with thereaction tubes adapted to transfer process fiuid from said reactiontubes to said quench chamber containing quench fluid.

10. The process of claim 8 wherein the quench tubes are adapted to ejectthe process fluid into a body of coolant liquid so as to quench theprocess fluid by direct heat exchanger with said coolant,

References Cited UNITED STATES PATENTS NORMAN YUDKOFF, Primary ExaminerD. EDWARDS, Assistant Examiner US. Cl. X.R.

